Make limit on scroll-margin variable

* src/xdisp.c (maximum-scroll-margin): New variable.
* lisp/cus-start.el: Make it customizable.
* etc/NEWS: Mention it.
* doc/emacs/display.texi (Auto Scrolling):
* doc/lispref/windows.texi (Textual Scrolling): Document it.
* src/window.c (window_scroll_pixel_based): Use it instead of hardcoding
division by 4 (Bug #5718).

6 files changed, 51 insertions, 9 deletions

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index c6e990d9082..15c700892bc 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -285,13 +285,17 @@ multiple variables, the order of priority is:
 @code{scroll-up-aggressively} / @code{scroll-down-aggressively}.
 
 @vindex scroll-margin
+@vindex maximum-scroll-margin
   The variable @code{scroll-margin} restricts how close point can come
 to the top or bottom of a window (even if aggressive scrolling
 specifies a fraction @var{f} that is larger than the window portion
-between the top and the bottom margins).  Its value is a number of screen
-lines; if point comes within that many lines of the top or bottom of
-the window, Emacs performs automatic scrolling.  By default,
-@code{scroll-margin} is 0.
+between the top and the bottom margins).  Its value is a number of
+screen lines; if point comes within that many lines of the top or
+bottom of the window, Emacs performs automatic scrolling.  By default,
+@code{scroll-margin} is 0.  The effective margin size is limited to a
+quarter of the window height by default, but this limit can be
+increased up to half (or decreased down to zero) by customizing
+@code{maximum-scroll-margin}.
 
 @node Horizontal Scrolling
 @section Horizontal Scrolling
diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index 6f3de0c8a0e..affa28c9202 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -3924,6 +3924,21 @@ redisplay scrolls the text automatically (if possible) to move point
 out of the margin, closer to the center of the window.
 @end defopt
 
+@defopt maximum-scroll-margin
+This variable limits the effective value of @code{scroll-margin} to a
+fraction of the current window line height.  For example, if the
+current window has 20 lines and @code{maximum-scroll-margin} is 0.1,
+then the scroll margins will never be larger than 2 lines, no matter
+how big @code{scroll-margin} is.
+
+@code{maximum-scroll-margin} itself has a maximum value of 0.5, which
+allows setting margins large to keep the cursor at the middle line of
+the window (or two middle lines if the window has an even number of
+lines).  If it's set to a larger value (or any value other than a
+float between 0.0 and 0.5) then the default value of 0.25 will be used
+instead.
+@end defopt
+
 @defopt scroll-conservatively
 This variable controls how scrolling is done automatically when point
 moves off the screen (or into the scroll margin).  If the value is a
diff --git a/etc/NEWS b/etc/NEWS
index ddd40fa8535..617f39f9b4c 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -307,6 +307,11 @@ local part of a remote file name.  Thus, if you have a directory named
 "/~" on the remote host "foo", you can prevent it from being
 substituted by a home directory by writing it as "/foo:/:/~/file".
 
++++
+** The new variable 'maximum-scroll-margin' allows having effective
+settings of 'scroll-margin' up to half the window size, instead of
+always restricting the margin to a quarter of the window.
+
 
 * Editing Changes in Emacs 26.1
 
diff --git a/lisp/cus-start.el b/lisp/cus-start.el
index a790419b86f..51c43c7d21a 100644
--- a/lisp/cus-start.el
+++ b/lisp/cus-start.el
@@ -511,6 +511,7 @@ since it could result in memory overflow and make Emacs crash."
              (scroll-step windows integer)
              (scroll-conservatively windows integer)
              (scroll-margin windows integer)
+             (maximum-scroll-margin windows float "26.1")
              (hscroll-margin windows integer "22.1")
              (hscroll-step windows number "22.1")
              (truncate-partial-width-windows
diff --git a/src/window.c b/src/window.c
index 235c3c1ade8..ba03780f3df 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4801,11 +4801,20 @@ window_scroll_margin (struct window *window, enum margin_unit unit)
     {
       int frame_line_height = default_line_pixel_height (window);
       int window_lines = window_box_height (window) / frame_line_height;
-      int margin = min (scroll_margin, window_lines / 4);
-      if (unit == MARGIN_IN_PIXELS)
-        return margin * frame_line_height;
-      else
-        return margin;
+
+      double ratio = 0.25;
+      if (FLOATP (Vmaximum_scroll_margin))
+        {
+          ratio = XFLOAT_DATA (Vmaximum_scroll_margin);
+          ratio = max (0.0, ratio);
+          ratio = min (ratio, 0.5);
+        }
+      int max_margin = min ((window_lines - 1)/2,
+                            (int) (window_lines * ratio));
+      int margin = clip_to_bounds (0, scroll_margin, max_margin);
+      return (unit == MARGIN_IN_PIXELS)
+        ? margin * frame_line_height
+        : margin;
     }
   else
     return 0;
diff --git a/src/xdisp.c b/src/xdisp.c
index 8a450b7a8a4..134ef6c6196 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -31520,6 +31520,14 @@ Recenter the window whenever point gets within this many lines
 of the top or bottom of the window.  */);
   scroll_margin = 0;
 
+  DEFVAR_LISP ("maximum-scroll-margin", Vmaximum_scroll_margin,
+    doc: /* Maximum effective value of `scroll-margin'.
+Given as a fraction of the current window's lines.  The value should
+be a floating point number between 0.0 and 0.5.  The effective maximum
+is limited to (/ (1- window-lines) 2).  Non-float values for this
+variable are ignored and the default 0.25 is used instead.  */);
+  Vmaximum_scroll_margin = make_float (0.25);
+
   DEFVAR_LISP ("display-pixels-per-inch",  Vdisplay_pixels_per_inch,
     doc: /* Pixels per inch value for non-window system displays.
 Value is a number or a cons (WIDTH-DPI . HEIGHT-DPI).  */);